CORRELATION BETWEEN SLOWLY REPAIRABLE DOUBLE-STRAND BREAKS AND THERMAL RADIOSENSITIZATION IN THE HUMAN HELA S3 CELL-LINE

The effect of heat on double-strand breaks (dsb) repair was compared w ith thermal radiosensitization using HeLa S3 cells. Cells were exposed to a combined treatment of X-irradiation followed by heat (44 degrees C, 0.5 h) separated by time intervals up to 8h. DNA dsb were measured by PFGE and survival by the colony forming assay. In non-heated HeLa S3 cells repair of dsb was biphasic with the majority of breaks being repaired fast with a half-lime of 14 min and only a minority were repa ired slowly with a half-time of 130 min. Heat applied immediately afte r irradiation was found to cause an increase in both half-times but ma inly to result in an increased fraction of slowly repairable dsb. The latter effect was shown to result from the formation of additional dsb . The number of additional dsb declined when irradiation and heat were separated by an interval at 37 degrees C with a half-time of 120 +/- 30 min. This half-time was similar to the half-time of 100 +/- 20 min found for the loss of thermal radiosensitization studied for the same protocol. Both processes were recently found also to correlate in CHO cells but occurred much faster in rodent cells than in the human HeLa S3 cells used in the current study. These results show that in human c ells, unlike previously suggested on the basis of rodent cells, therma l radiosensitization is still a substantial contributor to the killing efficacy of a combined treatment, even when irradiation and heat are separated by a time interval of 4h.